The present invention pertains to a downhole energy absorbing system for generation of electrical power, and more particularly to an apparatus for absorbing at least a portion of the vibrational energy from a drill string and converting such vibrational energy into electrical energy.
The problem of developing effective downhole electrical power supplies has existed in the mining and petroleum industries since the advent of downhole electrically operated devices. To a great extent, the development of electrically operated downhole equipment has been stymied by the lack of an effective power supply which will operate within the restrictive limitations of the downhole environment. In the past, great reliance has been placed on batteries for downhole power but the environment of a wellbore, particularly with respect to high temperatures and pressures, as well as space limitations, all mitigate against the use of batteries, especially where sustained power is required. As a result of the ineffectiveness of battery power for many such operations, most measuring techniques, especially during drilling operations, require the cessation of drilling while electrically operated apparatus is lowered into the borehole on an armored cable having one or more electrical conductors.
With the advent of deeper drilling and increased drilling activity offshore and in hostile surface environments, the costs of drilling have escalated substantially. Therefore, any operation which requires the cessation of drilling in order to be performed, such as lowering a cable into the borehole, is done at a great expense. In addition, because of the great expense of present drilling operations, the need has increased for obtaining real time data concerning downhole conditions, while drilling is progressing.
As a result, a great deal of development activity in the petroleum industry has been directed toward various telemetry systems for transmitting downhole data, relating to parameters measured while drilling, to the surface. Except for special circumstances, current methods of transmitting real time data in measure-while-drilling (MWD) systems have been marginally effective. Examples of important measurements to be made during drilling include rotation rate, penetration rate, torque, borehole fluid invasion, bit wear, formation parameters, etc. Presently in commercial use are mud pulse systems for telemetering data from the bit vicinity to the surface; however, these systems are expensive to use and have a low data rate.
There are basically four types of systems which show promise as communication and transmission systems in a borehole telemetry system. These are mud pressure pulse systems (mentioned above), electromagnetic methods, insulated conductor or hardwire systems, and acoustic methods. Developments in the last three methods have indicated the need to provide repeaters in the system in order to boost the signal as it is attenuated over a long and sometimes resistive conductor path. It appears that acoustic signals for example may travel effectively, under general drilling conditions, for 2 or 3 thousand feet before they are attentuated to unusable levels. The same is true of electromagnetic schemes. A hardwire system disclosed in U.S. Pat. No. 3,090,031 uses induction coupling between joints of pipe and electronic circuitry which necessitates the use of power sources at each coupling. In all of the proposed systems, excepting perhaps the mud pulse system, electrical power generation downhole to operate downhole circuits, including repeaters, presents a particularly difficult problem. The disadvantages of batteries have been discussed above. Present downhole generators typically rely on impellers or turbines stationed in the mud flow path to develop power. This tends to obstruct the full open bore in the pipe string, which may impede the insertion of equipment into the borehole through the drill pipe. In addition, such generators are adversely affected by the abrasive nature of drilling fluids which tend to wear flow channels and blades typically used in such devices and to damage bearings or the like.
In addition to the problem of effectively generating electrical power downhole in MWD systems, the vibrational environment afforded by the drill string is particularly harmful to electrical and mechanical hardwire systems associated with the measuring and telemetering of measured data to the surface in a drilling operation. As the formation is being drilled, an irregularly shaped hole bottom develops which causes the bit to rise and fall with rotation of the bit. This in turn causes the bit loading to vary. The variable load at the bit may be caused by other factors also. Thus conditions exist downhole which make the bit produce irregular demands on power, thus rendering the drill bit as a driver of drill string vibrations.
Additionally, the rotating drill string causes gyrations of the bottom hole assemblies above the drill bit which are transmitted to the bit in the form of load and torque variations. Thus, the drill string itself induces irregular power into the bit and thereby becomes the driver of drill string vibrations. In any event such vibrations cause fatigue forces to develop on the drill string and bit, increasing wear on the system as well as damaging mechanical and electrical components associated with a MWD system.
In order to minimize the effects of vibrational forces on a drill string and associated down hole assemblies, various shock absorbing assemblies have been developed for incorporation in the drill string, usually above the drilling bit, to isolate induced vibration, shock and impact loads from the drill string above the bit. Normally such shock absorbing subassemblies utilize a splined engagement between a mandrel and an elongated body, whereby drilling or rotational torque is transmitted through the splined engagement between the mandrel and the body. The splined section also permits longitudinal movement of the body with respect to the mandrel, to apply impact or longitudinal vibrational loads to a shock absorbing element, such as a spring system, in the subassembly. A shock absorbing subassembly of this type is disclosed in this inventor's U.S. Pat. No. 4,246,765 dated Jan. 27, 1981.
Keeping in mind the dual problems of dampening vibrations in a drill string to prevent fatigue to the drilling and instrumentation hardware in a drilling system and the need for a reliable downhole power supply, it is an object of the present invention to provide a new and improved downhole assembly for absorbing at least a portion of the vibrational energy occurring on a drill string and converting such absorbed energy into electrical energy to power downhole electrical devices.